Page:Popular Astronomy - Airy - 1881.djvu/41

Rh and he then listens to the clock, which beats seconds—in that manner he gets the hour, the minute, the second, and the fraction of a second, at which that bright star passes every wire, and by taking the mean or average of these, he finds the time at which the star passes the meridian. He looks again, and he sees a planet coming into the field of view. He directs his telescope to that planet, and in like manner he gets the time by the clock at which that planet passes the meridian—the hour, the minute, the second, and the fraction of a second. He sees another star. The telescope is moved to the proper position he notes the time in the same manner, and he finds the hour, the minute, the second, and the fraction of a second, as before. Another star comes in the same way. Such is the duty which a transit-observer has to perform—the watching of objects which are passing the meridian in endless succession. He has this instrument, which is confined in its motions to the meridian, and which admits of no other motion; and the clock, by which he notes the hour, the minute, the second, and the fraction of a second; by the use of the various wires, he observes the time at which the object passes each wire; and by taking the mean of all, he finds very accurately the time at which the object passes the meridian: such are the duties of the transit-observer.

The next thing is to ascertain the elevation of the object when it passes the meridian. Now before we enter upon the use of the Mural Circle, I must offer a word or two upon Geometry. I dare say everybody here, like myself, has in his time, studied books containing measures—so many barleycorns make an inch, so many inches make a foot, so many feet make a yard, etc., as well as so many yards make a mile,